Alpha Red A and B Flashing Fault Code: Causes, Fixes & Repair Costs
What does the Alpha Red A and B Flashing fault code mean?
The Red A and B indicator lights on Alpha boilers use their flash pattern to communicate three distinct fault conditions. **A and B flashing simultaneously (together at the same time)** indicates a flue blockage or fan fault — the boiler has detected a problem with flue gas extraction and shut down as a safety measure. **A and B flashing alternately (one after the other, taking turns)** indicates that system pressure has dropped too low, typically below 1 bar, and the boiler needs re-pressurising before it will operate again. **A flashing while B stays lit continuously (B steady)** points to a pump fault or restricted water flow through the system. In all three cases the boiler will remain locked out until the underlying problem is resolved. Identifying the correct flash pattern before attempting any fix is essential, as each pattern points to a completely different root cause.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Low system pressure (A and B alternating) Common
Pressure has dropped below the boiler's minimum threshold, often around 0.5 bar or lower. This is one of the most frequent causes of this fault. Pressure loss can happen gradually through micro-leaks, after bleeding radiators, or following a pressure relief valve discharge. The boiler pressure gauge will typically be sitting below 1 bar and outside the green zone.
- Fan failure or wiring fault (A and B simultaneous) Common
The PCB requires a confirmed signal from the fan before allowing the boiler to fire. If the fan motor has failed, seized, or a wiring connection to the fan has become loose or corroded, that signal never arrives and the boiler locks out to prevent unsafe combustion. Fan motors on older boilers are particularly prone to wear.
- Flue blockage (A and B simultaneous) Sometimes
A physical obstruction in the flue — such as a bird's nest, leaves, or debris — prevents combustion gases from escaping. The boiler detects the resulting pressure change and shuts down. A flue without a guard on a vertical terminal is particularly vulnerable. In some cases a flue section can come apart internally and partially block itself.
- Pump fault or seized pump (A flashing, B steady) Sometimes
If the circulating pump has seized, is running too slowly, or has an electrical fault, water cannot move around the system at the required rate. The boiler detects restricted flow and locks out. Pumps can seize after a period of inactivity — for example, at the start of the heating season — or wear out over time.
- Trapped air causing restricted flow (A flashing, B steady) Sometimes
A significant air pocket in the system can reduce water circulation enough to trigger the flow-fault lockout. This is more likely after system work, a recent power cut, or if radiators have not been bled for some time.
- PCB fault Rare
In rarer cases the PCB itself may misread sensor signals or develop an internal fault that causes it to report a fan, flow, or pressure error incorrectly. This is generally a diagnosis of exclusion once other causes have been ruled out.
How to fix it
- Identify the exact flash pattern before doing anything else DIY safe
Stand in front of the boiler and watch the Red A and B lights carefully. Are they flashing on and off together at the same time (simultaneous)? Are they taking turns — one on while the other is off (alternating)? Or is B glowing steadily while only A flashes? Write down what you see. The correct fix depends entirely on the pattern, so getting this right first saves time and avoids unnecessary steps.
- If A and B are alternating — check the pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If the needle is below 1 bar or sitting outside the green zone, low pressure is almost certainly the cause.
- Re-pressurise via the filling loop (low pressure only) DIY safe
Locate the filling loop — usually a short braided metal hose with one or two valves, found underneath the boiler. Slowly open the valve (or both valves) by turning a quarter-turn until you hear water entering the system. Watch the pressure gauge rise. Stop when the needle reaches about 1.2–1.5 bar and sits comfortably in the green zone, then close the valve(s) fully. Do not overfill — if the pressure goes above 1.5 bar, bleed a small amount of water from a radiator bleed valve until it drops back into range. If the boiler loses pressure again within days or weeks, you likely have a slow leak and should call an engineer.
- If A flashing / B steady — bleed the radiators to release trapped air DIY safe
Starting with the radiator furthest from the boiler, use a radiator bleed key to open the bleed valve a half-turn. Hold a cloth underneath. You will hear hissing as air escapes. Once a steady stream of water appears with no air, close the valve. Work through each radiator in the system. Check the boiler pressure gauge afterwards and top up via the filling loop if it has dropped below 1 bar.
- Attempt a boiler reset DIY safe
Once you have addressed the likely cause (topped up pressure, or bled radiators), try resetting the boiler. On most Alpha models there is a rotary selector switch on the front — rotate it to position 3 (reset) and hold it for no more than 2–3 seconds as specified in the manual, then release. On models with a dedicated reset button, press it once. Allow up to 30 seconds for the boiler to attempt a restart. Do not reset the boiler more than 2–3 times in quick succession — repeated resets without fixing the underlying cause can mask a more serious problem.
- If A and B flash simultaneously — do not attempt DIY repair; call a Gas Safe engineer Gas Safe engineer
A simultaneous flash pattern points to a fan or flue fault. Inspecting, testing, or replacing the fan, its wiring, or the flue system requires a Gas Safe registered engineer. Do not attempt to open the boiler casing or interfere with the flue. If you suspect the flue terminal is visibly blocked from outside (e.g. you can see a bird's nest), you can note this for the engineer, but leave all work to the professional.
- If the fault persists after re-pressurising or bleeding — call a Gas Safe registered engineer Gas Safe engineer
If the A and B alternating fault returns quickly after re-pressurising, a leak somewhere in the system needs to be found and repaired. If the pump fault pattern (A flashing, B steady) persists after bleeding radiators, the pump itself may need to be freed, serviced, or replaced. None of this work should be attempted by a homeowner. Contact a Gas Safe registered engineer who is familiar with Alpha boilers to carry out a full diagnosis.
Parts you may need
- Alpha boiler fan/flue fan motor · from £95
- Circulating pump (compatible replacement) · from £75
- Pressure relief valve · from £30
- Filling loop assembly · from £20
- PCB (Alpha CD/E-Tec series) · from £180
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£400, depending on the underlying cause.
Frequently asked questions
How do I tell whether Red A and B are flashing together or alternately on my Alpha boiler?
Watch the lights for a few seconds rather than glancing quickly. If both lights go on at the same moment and off at the same moment, they are flashing simultaneously — this points to a fan or flue fault. If one light comes on just as the other goes off, like a seesaw, they are alternating — this means low system pressure. Getting this distinction right is important because the fix for each is completely different.
Can I re-pressurise my Alpha boiler myself?
Yes — topping up system pressure via the filling loop is a straightforward task a homeowner can safely do. Find the braided filling loop underneath the boiler, slowly open the valve until the pressure gauge reaches around 1.2–1.5 bar in the green zone, then close it. If your boiler loses pressure again within a short time, this suggests a leak somewhere in the system, which will need a Gas Safe engineer to investigate and repair.
My Alpha boiler keeps showing this fault repeatedly — what does that mean?
A recurring fault usually means the root cause has not been properly resolved. For the low-pressure pattern, a slow system leak is the likely culprit — perhaps a weeping radiator valve, a leaking pipe joint, or a failing pressure relief valve. For fan or pump faults that return after resetting, the component itself may be failing and will eventually need replacing. Repeated resets without addressing the cause are not a long-term solution and can mask a worsening problem. A Gas Safe engineer should carry out a proper diagnosis.
How much does it cost to fix the fan or pump on an Alpha boiler?
For a fan replacement the typical total cost including parts and labour is roughly £250–£400. A pump replacement is usually in a similar range, around £200–£350 all in, depending on the specific pump required and the engineer's call-out rate in your area. PCB replacement, if needed, tends to cost more — typically £300–£500 — and is worth weighing against the age and overall condition of the boiler. For a boiler that is over 10–12 years old, it may be worth getting a new boiler quote alongside the repair quote.